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Studying Parton Energy Loss in Heavy-Ion Collisions via Direct-Photon and Charged-Particle Azimuthal Correlations

The STAR collaboration
Phys.Rev.C 82 (2010) 034909, 2010.

Charged-particle spectra associated with direct photon ($\gamma_{dir} $) and $\pi^0$ are measured in $p$+$p$ and Au+Au collisions at center-of-mass energy $\sqrt{s_{_{NN}}}=200$ GeV with the STAR detector at RHIC. A hower-shape analysis is used to partially discriminate between $\gamma_{dir}$ and $\pi^0$. Assuming no associated charged particles in the $\gamma_{dir}$ direction (near side) and small contribution from fragmentation photons ($\gamma_{frag}$), the associated charged-particle yields opposite to $\gamma_{dir}$ (away side) are extracted. At mid-rapidity ($|\eta|<0.9$) in central Au+Au collisions, charged-particle yields associated with $\gamma_{dir}$ and $\pi^0$ at high transverse momentum ($8< p_{T}^{trig}<16$ GeV/$c$) are suppressed by a factor of 3-5 compared with $p$ + $p$ collisions. The observed suppression of the associated charged particles, in the kinematic range $|\eta|<1$ and $3< p_{T}^{assoc} < 16$ GeV/$c$, is similar for $\gamma_{dir}$ and $\pi^0$, and independent of the $\gamma_{dir}$ energy within uncertainties. These measurements indicate that the parton energy loss, in the covered kinematic range, is insensitive to the parton path length.

  • Figure 2

    Data from Figure 2


    The $z_{T}$ dependence of $\pi^{0}-h^{\pm}$ near side and away-side associated particle yields. The errors denoted 'syst' are systematic errors correlated...

  • Figure 3 (upper panel)

    Data from Figure 3 (upper panel)


    The $z_{T}$ dependence of away-side associated-particle yields for $\pi^{0}-h^{\pm}$ triggers and $\gamma_{dir}$ triggers. The errors denoted 'syst' are systematic errors...

  • Figure 3 (lower panel)

    Data from Figure 3 (lower panel)


    The $z_{T}$ dependence $I_{AA}$ for $\pi^{0}-h^{\pm}$ triggers and $\gamma_{dir}$ triggers. The errors denoted 'syst' are systematic errors correlated in $z_{T}$....

  • Figure 4

    Data from Figure 4


    $I_{AA}$ as a function of $p_{T}$ for $\gamma_{dir}$ triggers, measured in 0-10% Au+Au collisions. The associated charged particles have $z_{T}...

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